Atoms, very small particles, are made of even
smaller particles (a particle is just a small
amount of matter that stay "clumped" together, and
that can be characterized by mass, volume,
density, and so on).

Every atom has electrons, which are
particles that have negative electrical charges;
every atom also has protons, which are
particles that have positive electrical charges.

In terms of mass (how much "stuff" there is
in an object), a proton has approximately
1800 times the mass of an electron, but its
electrical charge is exactly equal to the charge
of the electron, just positive instead of
negative.

With the exception of some hydrogen atoms,
every atom also has neutrons, which are
particles with the same mass as protons but no
charge, positive or negative. Many atoms have
the same number of neutrons and protons, and
electrically neutral (no total positive or
negative charges) atoms all have the same number
of electrons as protons. For example, an
electrically neutral hydrogen atom has one proton
and one electron; an electrically neutral helium
atom has two protons, two neutrons, and two
electrons.

Protons and neutrons are made of even
smaller particles called quarks. All these
particles interact with one another and are held
together by different forces. Part of the mystery
of particle physics is that an atom has a
radius about 100,000 times the radius of a proton,
which means that atoms are actually mostly
empty space! Yet atoms are one of the
fundamental particles that make things what they
are - the properties of different atoms make salt
different from baking soda and different from
water. It is this mystery of physics that drives
some physicists to study the nature of these
particles.